It's a triumph that's been 84 years in the making and, despite a concerted effort by all involved to stay humble and on-message during their movement's finest hour, few members of Egypt's Muslim Brotherhood can hide their exhilaration.

"These elections are a historic milestone for us and they are a historic milestone for Egypt," said Amr Darrag, secretary general of the Brotherhood's Freedom and Justice party in Giza. "We've not had such free elections since 1952, so it's a great moment for the nation," he said. "The Egyptian people really intend to seize this moment and secure the position for Egypt that it so clearly deserves."

The FJP will be the largest force in the country's new parliament when it opens for business on 23 January, almost exactly a year on from the beginning of the revolution that would eventually topple the Brotherhood's tormentor-in-chief, Hosni Mubarak.

After decades of having to play by the rules of your oppressors, it's a giddying feeling to start writing them yourself. Seham al-Gamal, a Muslim Brotherhood activist who ran unsuccessfully for parliament in 2010, remembers the old game only too well. "Politics was based on corruption, and only groups that accepted this corruption flourished in the political arena," she said in an interview with one of the Brotherhood's slick, regularly updated, bilingual websites this week.

The movement's online presence contrasts sharply with that of parliament itself: Egypt's legislature boasts two homepages, one of which was last updated in 2001 and the other is permanently offline. Now both the institution and the party that will dominate it are preparing for heady times. "[In the last election] there was scandalous vote rigging in favour of [Mubarak's] dissolved NDP," said al-Gamal. "[Now] we plan to combat corruption in its entirety."

In just over a week, Gamal will walk through the doors of Egypt's almost century-old parliament building and take her seat as elected representative of the Nile Delta town of Mansoura. She will be alongside at least 192 colleagues from the FJP, marking a milestone of epic proportions for a movement which, as recently as 12 months ago, was headquartered on the second floor of a shabby apartment block and had much of its senior leadership behind bars. Today the FJP boasts a gleaming six-storey home in an upmarket Cairo suburb, its figureheads are feted by international policymakers, and its MPs are set to dominate the country's first post-Mubarak people's assembly.

But although it's easy to see the outcome of Egypt's elections as an unqualified success for the Brotherhood, the streets around the parliament building tell a more complex story. Entrances to the north and east are blocked off with hastily erected giant granite barriers, each presiding over a carpet of concrete rubble and twisted metal, the residue of recent street-fighting between revolutionaries and the security forces that left dozens dead and several thousand injured.

All other approaches to the building, which sits on a normally busy thoroughfare just off Tahrir Square, are clad in barbed wire and ad hoc security checkpoints; amid a row of pristine government cars sits one vehicle that has been gutted by a petrol bomb. Nearby walls and floors are blanketed in graffiti marking the dates of last year's struggles, when the roads became a violent battleground between those who maintain a grip on the formal sphere of politics and those who have opted to remain outside it.

The message is that, despite the election of a new parliament, the era of street revolt is far from over and the fragile legitimacy of the military junta's supposedly democratic institutions is yet to be accepted by all. As Gamal begins her first day at work, she will be greeted on her way in by a four-foot sprawl of spray paint, heralding 2012 as the start of Egypt's second revolution.

It is against this uncertain backdrop that the lower house of parliament will assemble next week, raising the question of what the legislative body can hope to achieve in such a volatile climate, and how the Brotherhood will manage its new-found authority within it.

Although final confirmation of the results won't emerge until the weekend, 85% of the seats have been decided and the remaining contests will not sway the general balance of the parties. At present the Brotherhood is on course to claim 45% of MPs, with the more conservative Salafists likely to gain 25% of the total. Secular liberals such as the venerable al-Wafd party and the newly formed Egyptian bloc should secure 15% between them, with the last few seats picked up by other Islamist parties, nationalistic "remnants" of the Mubarak regime, and leftwing political forces.

According to the current "transition timetable", parliament will quickly appoint a special constitutional assembly and begin working on a new constitution, with a referendum on the new document to be held by April and presidential elections to follow in June.

It's a breakneck – some would say slapdash – pace of change, leaving critics concerned that a new political system that leaves the power of the military and other elites essentially untouched will quickly become entrenched, thwarting any deeper, long-term struggles for economic and social justice.

The Brotherhood, time-worn experts at political survival and firm believers in a cautious "participation, not domination" approach, has ascended the political elevator just as the country's contested future remains hanging in the balance, and the group stands torn between enjoying its electoral spoils and retaining credibility with its supporters by not bowing too subserviently to the junta.

"The movement is entering uncharted waters," says Nathan Brown, an Egypt expert at George Washington University who argues that despite parliamentary victory, it is not yet clear what exactly the Brotherhood has won. "A number of features of Egypt's transition process suggest that however many votes the Brotherhood attracts in the parliamentary elections, it will have a strong, but hardly a dominant voice, for the time being. 'Participation, not domination' may continue to work for now because of the hazy rules governing a country in transition."

Mindful of the trap that lies before it – one that twins soaring public expectations with very few concrete powers to deliver reform – the Brotherhood is already manoeuvring to give itself cover, abandoning its earlier call for Egypt to become a parliamentary democracy and shifting its backing to a presidential system, alongside a promise that the FJP will not run a candidate for the presidency.

The emphasis is on partnership and coalition with other forces, including liberals, but the oft-repeated mantra of Egypt's problems being "too big for one party" has been complicated by the FJP's massive haul of seats. This has left other parties viewing the Brotherhood as a threat and consequently wary of linking up with it.

The Brotherhood is also treading a fine line on its policy programme, advocating a vision of market-friendly reform aimed at boosting living standards and wiping out corruption while pushing cultural and moral issues in the background – a move reassuring to secular Egyptians and international audiences fearful of a lurch towards religious conservatism, were it not for the worry that leaving such political space vacant would allow the hardline Salafists to move in.

But perhaps the biggest challenge for the Muslim Brotherhood and the parliament it now leads will be to win over those who distrust this junta-curated organ of official "democracy", and move Egypt's political process out of the streets and into a formal arena instead.

Philip Rizk, an Egyptian activist and film-maker, echoes the sentiments of many revolutionaries when he argues that the recent elections are designed to stifle meaningful change. "The ballot was used in two ways," he claims. "Firstly there was a very specific discourse from the authorities, from the Supreme Council of the Armed Forces and the transitional government, saying 'stop protesting and go out and vote'. It was a propaganda effect aimed at altering people's perceptions of change. Secondly, the vote was used as a cover for direct physical attacks on the protest movement that were carried out with exceptional violence."

The Brotherhood, Rizk argues, has not been a radical or revolutionary player in the dramatic upheaval this nation has witnessed over the past year, and both its commitment to neoliberal capitalism and its alleged electoral violations – there have been accusations of illegal campaigning and voter intimidation in polling stations, all of which are denied by the FJP – have left it "barely distinguishable" from Mubarak's NDP. "The most significant change that has taken place in Egypt is the one that has taken place inside people," says Rizk. "Not the whole population, but a large proportion of it, have started analysing and thinking and protesting, both within their spheres of daily activity and more centrally in urban squares like Tahrir, as well as other cities like Alexandria and Suez. Elections can come and go but I don't think that is going to shift: the bubble of complacency and fear has been broken."

How the new parliamentarians respond to such a change remains to be seen. In a shift away from some of its more critical rhetoric towards protesters, leaders such as Darrag are reaching out to revolutionaries, acknowledging their concerns and admitting that the onus is on parliament (and by extension, the Brotherhood) to prove that it really can implement the demands of the revolution and sweep away the old elites. "To be honest, most of the revolution's demands have not yet been met and many are suspicious that remnants of the old regime remain in power and are controlling things from behind the scenes, not necessarily in the realm of higher politics but also other decision-making bodies," he argues. "People need to see the institutions that have been established – the parliament, and later the presidency – act in their interests and secure real achievements before they start believing that they have really made a difference."

But with so many other unpredictably factors in play, the Brotherhood has a long way to go before it can build a genuinely national consensus and confidently get a handle on the levers of political power. As Nathan Brown concludes: "In the Egypt of 2012, the Brotherhood's leaders will have to answer questions that have never been asked before."